The entire disclosure of Japanese Patent Applications No. Hei 11-306895 filed on Oct. 28, 1999 including the specification, drawings and abstract, is incorporated herein by reference in its entirety.
1. Field of the Invention
This invention relates to a pulse tube refrigerator and, more particularly, to a pulse tube refrigerator for cryogenic refrigeration.
2. Description of the Related Art
A pulse tube refrigerator is attractive as a cryogenic refrigerator. The pulse tube refrigerator refrigerates a working fluid by oscillating the working fluid therein, by shifting the phase of the pressure change and the position change.
Various structures for a pulse tube refrigerator of this kind have been proposed. For instance, the one introduced by M. David et al, in Cryogenics, Vol. 30, (1990), P. 262-266, and illustrated in the block diagram of FIG. 4. A pulse tube refrigerator 60 of this structure comprises a pressure oscillator 61, a refrigerating portion 62, a middle pressure buffer tank 63 and a middle pressure buffer side valve 64.
The pressure oscillator 61 generating pressure oscillation to the working fluid filled in the pulse tube refrigerator 60 comprises a compressor 71, a high pressure valve 72 and a low pressure valve 73. An outlet port 71a of the compressor 71 is connected to the refrigerating portion 62 via the high pressure valve 72. An inlet port 71b of the compressor 71 is connected to the refrigerating portion 62 via the low pressure valve 73. The pressure oscillator 61 generates pressure oscillations in the working fluid in the refrigerating portion 62 of the pulse tube refrigerator 60 by controlling the opening and closing of the high pressure valve 72 and the low pressure valve 73 at a predetermined timing. The maximum pressure Ph which is an output pressure of the compressor 71 is set at 2 MPa, and the minimum pressure P1 of an input pressure of the compressor 71 is set at 1 MPA.
The refrigerating portion 62 comprises a regenerator 74, a low temperature heat exchanger 75, a pulse tube 76 and a high temperature heat exchanger 77 connected in series, inline.
A hot end 74a of the regenerator 74 is connected to the pressure oscillator 61. A cold end 74b is connected to the low temperature heat exchanger 75. The regenerator 74 gradually refrigerates the working fluid while the working fluid moves therethrough towards the low temperature heat exchanger 75 side, and gradually heats the working fluid moving therethrough towards the pressure oscillator 61 side.
The low temperature heat exchanger 75 connected to the cold end 74b of the regenerator 74 generates a low temperature. In order to effectively remove the heat of a device to be refrigerated, such as an electronic device, in contact with the low temperature heat exchanger 75, the low temperature heat exchanger 75 is provided with a number of holes regularly formed along the flow direction of the working fluid.
The pulse tube 76 connected to the low temperature heat exchanger 75 is formed by a hollow tube having a cold end 76a on the low temperature heat exchanger 75 side and a hot end 76b on the high temperature heat exchanger 77 side. The pulse tube 76 is made of a material with low heat conductivity in order to prevent the transfer of the heat generated by the oscillation from the hot end 76b side to the low temperature heat exchanger side.
The high temperature heat exchanger 77 connected to the pulse tube 76 includes a number of holes regularly arranged along the flowing direction of the working fluid. The high temperature heat exchanger 77 refrigerates the hot end 76b side by releasing the heat of the working fluid flowing therethrough to outside thereof. The high temperature heat exchanger 77 is connected to the middle pressure buffer side valve 64.
The middle pressure buffer side valve 64 is provided between the high pressure heat exchanger 77 of the refrigerating portion 62 and the middle pressure buffer tank 63. A phase lag (phase difference) between pressure oscillation and displacement of the working fluid in the pulse tube 76 is adjusted by opening and closing the middle pressure buffer side valve 64 at a predetermined timing. The volume of the middle pressure buffer tank 63 is much larger than that of the refrigerating portion 62 of the pulse tube refrigerator 60. The pressure of the working fluid in the middle pressure buffer tank 63 is kept at an approximately average pressure (1.5 MPa) of the maximum pressure Ph (output pressure) and the minimum pressure P1 (input pressure) of the compressor 71.
Basic operation of the pulse tube refrigerator 60 will be explained as follows, referring to FIG. 5. Operation in one cycle of the pulse tube refrigerator 60 consists of four stages (a) to (d), explained as follows. Each stage is defined in accordance with the respective opening and closing condition of the high pressure valve 72, the low pressure valve 73 and the middle pressure buffer side valve 64.
FIG. 5 is a diagram showing the opening and the closing conditions of the high pressure valve 72, the low pressure valve 73 and the middle pressure buffer side valve 64, and the pressure condition in the pulse tube 76 at each stage (a) to (d) in one cycle of the pulse tube refrigerator 60. In FIG. 5, each bold line for the high pressure valve 72, the low pressure valve 73 and the middle pressure buffer side valve 64 respectively shows the opening condition, and each fine line shows the closing condition of the valves 72, 73, and 64. The operation of the pulse tube refrigerator at each stage (a) to (d) in one cycle will be explained as follows.
First stage (a) (First Half of Compression Stage)
The state in which the low pressure valve 73 is kept closed and the high pressure valve 72 is kept closed continuously from the previous stage (Second Half of Expansion Stage), whereas the middle pressure buffer control valve 64 is kept open. In this state, the pressure in the pulse tube 76 increases from the minimum pressure P1 to the average pressure Pm (the pressure in the middle pressure buffer tank 63).
Second stage (b) (Second Half of Compression Stage)
The state in which the middle pressure buffer side valve 64 is kept closed and the low pressure valve 73 is kept closed continuously from the previous stage (First Half of Compression Stage), whereas the high pressure valve 72 is kept open. In this state, the pressure in the pulse tube 76 increases from the average pressure Pm to the maximum pressure Ph.
Third stage (c) (First Half of Expansion Stage)
The state in which the high pressure valve 72 is kept closed and the low pressure valve 73 is kept closed continuously from the previous stage (Second Half of Compression Stage), whereas the middle pressure buffer side valve 64 is kept open. In this state, the pressure in the pulse tube 76 falls from the maximum pressure Ph to the average pressure Pm (the pressure in the middle pressure buffer 63). Accordingly, the reduction of the pressure causes the adiabatic expansion of the working fluid in the pulse tube 76 to lower the temperature.
Fourth stage (d) (Second Half of Expansion Stage)
The state in which the middle pressure buffer control valve 64 is kept closed and the high pressure valve 72 is kept closed continuously from the previous stage (First Half of Expansion Stage), whereas the low pressure valve 73 is kept open. In this state, the pressure in the pulse tube 76 falls from the average pressure Pm to the minimum pressure P1. Accordingly, the pressure decrease causes further adiabatic expansion of the working fluid in the pulse tube 76 to further lower the temperature.
The foregoing stages (a) to (d) comprise one cycle, and by repetition of this cycle the working fluid repeats movement towards one side to release the heat at the high temperature heat exchanger 77 and towards the other side to absorb the heat at the low temperature heat exchanger 75. The pulse tube refrigerator 60 thus generates a cryogenic temperature at the low temperature heat exchanger 75 of the refrigerating portion 62.
In the pulse tube refrigerator 60, the opening operation of the high pressure valve 72 at stage (b) and the opening operation of the low pressure valve 73 at stage (d) must be performed at a large pressure difference (the differential pressure between the maximum pressure Ph and the average pressure Pm or the differential pressure between the average pressure Pm and the minimum pressure P1). Accordingly, the losses generated due to the opening of the valves under different pressure condition, which is a thermodynamically irreversible process (valve loss), has been high. The generation of this high valve loss leads to an increase of the load of the compressor 71, which decreases the refrigeration efficiency of the pulse tube refrigerator 60.
Japanese Patent No. 2553822 addresses the irreversible process problem (the generation of the valve loss) due to the opening operation of the high pressure valve 72 and the low pressure valve 73. FIG. 6 is a block diagram of the pulse tube refrigerator disclosed in this Japanese Patent. As shown in FIG. 6, the pulse tube refrigerator 80 comprises a low pressure buffer tank 81, a low pressure buffer side valve 82, a high pressure buffer tank 83 and a high pressure buffer side valve 84, instead of the middle pressure buffer tank 63 and the middle pressure buffer side valve 64 included in the pulse tube refrigerator 60. Since the pressure oscillator 61 and the refrigerating portion 62 of the pulse tube refrigerator 80 and the pulse tube refrigerator 60 are identical, the same numerals are provided for the components thereof, and the explanation therefor will be omitted.
The low pressure buffer side valve 82 provided between the high temperature heat exchanger 77 of the refrigerating portion 62 and the low pressure buffer tank 81 adjusts the phase lag between the pressure oscillation and displacement of the working fluid in the pulse tube 76 of the pulse tube refrigerator 80 by opening and closing at a predetermined timing. The volume of the low pressure buffer tank 81 is much larger than that of the refrigerating portion 62 of the pulse tube refrigerator 80. The pressure of the working fluid in the low pressure buffer tank 81 is set to a minimum pressure P1 (1 MPa).
The high pressure buffer side valve 84 provided between the high temperature heat exchanger 77 of the refrigerating portion 62 and the high pressure buffer tank 83 adjusts the phase lag between the pressure oscillation and displacement of the working fluid in the pulse tube 76 of the pulse tube refrigerator 80 by opening and closing at a predetermined timing. The volume of the high pressure buffer tank 83 is much larger than that of the refrigerating portion 62 of the pulse tube refrigerator 80. The pressure of the working fluid in the high pressure buffer tank 83 is set to a maximum pressure Ph (2 MPa).
Basic operation of the pulse tube refrigerator 80 will be explained as follows, referring to FIG. 7 and FIG. 8. The operation of the pulse tube refrigerator 80 includes four stages (a) to (d) in one cycle, explained as follows. Each stage is defined in accordance with each opening and closing condition of the high pressure valve 72, the low pressure valve 73, the low pressure buffer side valve 82, and the high pressure buffer side valve 84.
FIG. 7 is a diagram showing opening and closing conditions of the high pressure valve 72, the low pressure valve 73, the low pressure buffer side valve 82 and the high pressure buffer side valve 84, and the pressure condition in the pulse tube 76. FIG. 8 is a schematic view showing the distribution (volume) of the working fluid In the pulse tube 76 at stages (a) to (d) respectively. In FIG. 7, each bold line for the high pressure valve 72, the low pressure valve 73, the low pressure buffer side valve 82 and the high pressure buffer side valve 84 shows each opening condition thereof, and each fine line shows each closing condition thereof. In FIG. 8, Numeral I represents a block of the working fluid flowing into and flowing out from the compressor 71 at the cold end 76a of the pulse tube 76. Numeral II represents a block of the working gas constantly present in the pulse tube 76 in one cycle and functioning as a gas piston therein. Numeral III represents a block of the working fluid flowing into and out from the low pressure buffer 81 at the hot end 76b of the pulse tube 76. Numeral IV represents a block of the working fluid flowing into and out from the high pressure buffer 83 at the hot end 76b. In FIG. 8, the volume of the working fluid represented as blocks I to IV at each stage (a) to (d) is calculated according to the result of a numerical analysis assuming that the working gas in the pulse tube 76 achieves a complete adiabatic change. Accordingly, the volume change of the working fluid blocks I to IV in one cycle is approximate to the actual moving volume of the working fluid. The operation of the pulse tube refrigerator 80 at each stage in one cycle will be explained as follows.
First stage (a) (Compression Stage)
The state in which the low pressure valve 73 and the low pressure buffer side valve 82 are kept closed and the high pressure valve 72 is kept closed continuously from the previous stage (Low Pressure Transfer Stage), whereas the high pressure buffer side valve 84 is kept open. In this state, the working fluid in the high pressure buffer tank 83 (block IV) maintained at the maximum pressure Ph flows into the pulse tube 76 through the hot end 76b via the high pressure buffer side valve 84. Since the high pressure buffer tank 83 and the pulse tube 76 are in communication with each other via the high pressure buffer side valve 84, the pressure in the pulse tube 76 promptly increases from the minimum pressure P1 to the maximum pressure Ph.
Second stage (b) (High Pressure Transfer Stage)
The state in which the high pressure valve 72 is kept open and the high pressure buffer side valve 84 is kept open continuously from the previous stage (Compression Stage), whereas the low pressure valve 73 and the low pressure buffer side valve 82 are both kept closed continuously from the previous stage (Compression Stage). In this state, the working fluid from the outlet port 71a of the compressor 71 (block I) which is the maximum pressure Ph flows into the pulse tube 76 through the cold end 76a via the high pressure valve 72. In this case, since the pressure of the working fluid in the high pressure buffer tank 83 is slightly lower than the maximum pressure Ph, because the working fluid in the high pressure buffer tank 83 flowed out to the pulse tube 76 at the previous stage, the working fluid from the high pressure buffer tank 83 (block IV) is forced to return to the high pressure buffer tank 83 by the working fluid in the block I.
Third stage (c) (Expansion Stage)
The state in which the high pressure valve 72 and the high pressure buffer control valve 84 are kept closed and the low pressure valve 73 is kept closed continuously from the previous stage (High Pressure Transfer Stage), whereas the low pressure buffer side valve 82 is kept open. Since the low pressure buffer tank 81, whose pressure is maintained at the minimum pressure P1, and the pulse tube 76 are in communication with each other via the low pressure buffer control valve 82 in this state, the pressure in the pulse tube 76 promptly falls from the maximum pressure Ph to the minimum pressure P1. The working fluid in the pulse tube 76 adiabatically expanded by this pressure decrease to lower the temperature. In this case, the working fluid from the low pressure buffer tank 81 (block III) returns to the low pressure buffer tank 81 through the hot end 76b of the pulse tube 76 via the low pressure buffer side valve 82.
Fourth stage (d) (Low Pressure Transfer Stage)
The state in which the low pressure valve 73 is kept open and the low pressure buffer side valve 82 is kept open continuously from the previous stage (Expansion Stage), whereas the high pressure valve 72 and the high pressure buffer side valve 84 are both kept closed continuously from the previous stage (Expansion Stage). In this state, the working fluid in the pulse tube 76 flown from the outlet port 71a of the compressor 71 at the previous stages (block I) is absorbed into the inlet port 71b of the compressor 71 via the low pressure valve 73. Since the pressure of the working fluid in the low pressure buffer tank 81 is slightly higher than the minimum pressure P1 because the working fluid in the pulse tube 76 flowed in the low pressure buffer tank 81 at the previous stage, the working fluid in the low pressure buffer tank 81 (block III) f lows into the pulse tube 76 through the hot end 76b via the low pressure buffer side valve 82. The working fluid (block I) moved to the low temperature heat exchanger 75 conducts heat exchange therewith, and the condition returns to stage (a).
The foregoing stages (a) to (d) comprise one cycle, and this cycle is repeated to generate a cryogenic temperature at the low temperature heat exchanger 75 of the refrigerating portion 62 in the pulse tube refrigerator 80.
In the pulse tube refrigerator 80, since the opening operations of the high pressure valve 72 and the low pressure valve 73 at stages (b) and (d) are performed under a small differential pressure, the valve losses at stages (b) and (d) are reduced. However, since the opening operation of the high pressure buffer side valve 84 at stage (a) and the opening operation of the low pressure buffer side valve 82 at stage (c) are required to be performed under a large differential pressure (the differential pressure between the maximum pressure Ph and the minimum pressure P1), the generation of valve losses is high at stages (a) and (c). The valve losses caused by the opening operation of the high pressure buffer side valve 84 and the low pressure buffer side valve 82 increase the loading of the compressor 71, which reduces the refrigeration efficiency of the pulse tube refrigerator 80.
As shown in FIG. 8, each moving volume of the working fluid of block I (stage (b) to (d)), block III (stage (c) to (d)) and block IV (stage (a) to (b)) becomes large. Accordingly, the load of the compressor 71 is increased due to the increased moving volume of the working fluid in block I, block III and block IV. For the large moving volume of the working fluid in block I, block III and block IV, the heat loss in the pulse tube 76 due to the entropy flowing into the cold end 76a from the hot end 76b of the pulse tube 76, and the regenerating heat loss due to the entropy flowing from the hot end 74a to the cold end 74b, without being accumulated at the regenerator 74 increases, which reduces the refrigerating efficiency of the pulse tube refrigerator 80. It has been confirmed by the inventors that the reduction of the refrigeration efficiency of the pulse tube refrigerator 80 due to the increase of the heat loss or the regenerating heat loss in the pulse tube 76 is high at cryogenic temperatures (less than or equal to 77 K).
The increase of the moving volume of the working fluid flowing into and out from the high pressure buffer tank 83 at the hot end 76b of the pulse tube 76 (block IV) at stages (a) and (b) has the following cause. While the high pressure buffer side valve 84 is kept open at stage (a), the pressure in the pulse tube 76 at the minimum pressure P1 is required to be increased to the maximum pressure Ph by further supplying working fluid thereto from the high pressure buffer tank 83. While the high pressure buffer side valve 84 and the high pressure valve 72 are kept open at stage (b), the working fluid (block IV) is required to be supplied to the high pressure buffer tank 83 from the compressor 71 in order to maintain the maximum pressure Ph in the high pressure buffer tank 83. Accordingly, the moving volume of the working fluid (block IV) is increased.
The moving volume of the working fluid flowing into and flowing out from the low pressure buffer tank 81 at the hot end 76b of the pulse tube 76 (block III) at stages (c) and (d) is increased by the same reason. Accompanying the increase of the moving volume of the working fluid (block III, block IV), the moving volume of the working fluid flowing into and out from the compressor 71 at the cold end 76a of the pulse tube 76 (block I) is increased at stages (b) to (d).
Accordingly, an object of this invention is to reduce a valve losses in each cycle, to improve refrigeration efficiency of the pulse tube refrigerator.
To solve the foregoing problems, the pulse tube refrigerator of this invention includes a refrigerating portion comprising a regenerator, a low temperature heat exchanger, a pulse tube and a high temperature heat exchanger connected in series, inline. A pressure oscillator has a compressor, a high pressure valve and a low pressure valve, and generates pressure oscillations of the working fluid in the pulse tube by connecting an output port and an inlet port of the compressor to the regenerator via the high pressure valve and the low pressure valve respectively. A plurality of buffer tanks each have a different middle pressures level between the output pressure and the input pressure of the compressor, and are connected to the high temperature heat exchanger via respective buffer side valves for adjusting a phase lag between the pressure oscillation and displacement of the working fluid in the pulse tube.
Since a plurality of buffer tanks, each having a different pressure level predetermined as the middle pressures between the output pressure and the input pressure of the compressor, are connected to the high temperature heat exchanger via respective buffer side valves, when the opening state of each buffer side valve, the high pressure valve and the low pressure valve are arranged not to overlap one another in the order of a predetermined pressure controlling process (ascending, descending order) during the refrigeration cycle, each stage of the cycle is performed with a relatively small differential pressure between adjacent middle pressures. In consequence, the moving volume of the working fluid flowing into and out from the compressor at the cold end of the pulse tube, and the moving volume of the working fluid flowing into and out from each buffer tank at the hot end of the pulse tube, are reduced respectively in order to generate a predetermined pressure condition. Due to the reduction of the moving volume of the working fluid, the load of the compressor is reduced.
Due to the reduction of the moving volume of the working fluid, the heat loss in the pulse tube due to entropy flowing from the hot end towards the cold end of the pulse tube, and the regenerating heat loss due to entropy flowing from the hot end to the cold end without being reserved in the regenerator, are greatly reduced, which improves the refrigeration efficiency of the pulse tube refrigerator.
Due to the reduction of the moving volume of the working fluid, the volume size required for each buffer tank is reduced.
The valve losses due to the opening operation of the control valve under different pressure conditions, which is a thermodynamically irreversible process, are reduced as a whole by performing the opening operation of the control valves of each buffer, the compressor high pressure control valve and the compressor low pressure control valve under a relatively small differential pressure, which reduces the dynamic force load of the compressor.
In another aspect of the pulse tube refrigerator of this invention, the pulse tube refrigerator has two buffer tanks (a first buffer tank and a second buffer tank). Since two buffer tanks are provided, the volume size required for each buffer tank is reduced, to achieve a size reduction of the pulse tube refrigerator as a whole, while adding a minimum number of buffer tanks.
In a further aspect of the invention, the buffer tanks having a first middle pressure and a second middle pressure respectively comprise a first middle pressure buffer tank connected to the high temperature heat exchanger via a first middle pressure buffer side valve and a second middle pressure buffer tank connected to the high temperature heat exchanger via a second middle pressure buffer side valve. The high pressure valve, the low pressure valve, the first middle pressure buffer side valve and the second middle pressure buffer side valve are opened in the order of a predetermined pressure controlling process. Opening conditions of the high pressure valve, the low pressure valve, the first middle pressure buffer side valve and the second middle pressure buffer side valve are predetermined not to overlap one another.
Accordingly, each stage of a cycle is performed under the relatively small differential pressures of adjacent different middle pressures. In consequence, the moving volume of the working fluid flowing into and out from the compressor at the hot end of the pulse tube, and the moving volume of the working fluid flowing into and out from the first and the second middle pressure buffer tank at the hot end of the pulse tube, are reduced. Due to the reduction of the moving volume of the working fluid, the load of the compressor is reduced.
The heat loss and the regenerating heat loss in the pulse tube is greatly reduced by the reduction of the moving volume of the working fluid, and the refrigeration efficiency of the pulse tube refrigerator is improved.
The volume size required for the first and the second middle pressure buffer tanks is reduced by the reduction of the moving volume of the working fluid, which achieves a size reduction of the pulse tube refrigerator.
By opening the high pressure valve, the low pressure valve, the first middle pressure buffer side valve and the second middle pressure buffer side valve under the relatively small differential pressure, the valve losses are reduced as a whole, and the driving force required for the compressor is reduced.
According to a still further aspect of this invention, the pulse tube refrigerator includes a pulse tube having a hot end and a cold end, the compressor being in fluid communication with the cold end of the pulse tube, the first pressure buffer tank having the first pressure being in communication with the hot end of the pulse tube and the second pressure buffer tank having the second pressure being in communication with the hot end of the pulse tube. A working fluid includes a first gas block (block I) flowing into and out from the compressor at the cold end of the pulse tube, a second gas block (block III) functioning as a gas piston is constantly present in the pulse tube, a third gas block (block III) flowing into and out from the first pressure buffer tank at the hot end of the pulse tube and a fourth gas block (block IV) flowing into and out from the second pressure buffer tank at the hot end of the pulse tube. Means are provided for reducing the moving volume of the first gas block, the third gas block and the fourth gas block by reducing the differential pressure at each stage of the refrigeration cycle. The load of the compressor is thereby reduced.
Reduction of the moving volume of the first gas block, the third gas block, and the fourth gas block largely reduces the heat loss and the regenerating heat loss in the pulse tube, which improves the refrigeration efficiency of the pulse tube refrigerator.
Reduction of the moving volume of the first gas block, the third gas block, and the fourth gas block reduces the volume size required for the first and the second pressure buffer, which reduces the size of the pulse tube refrigerator.
Since the differential pressure at each stage in the refrigeration cycle is reduced, each valve provided with the pulse tube refrigerator is opened under a relatively small differential pressure, which reduces the valve losses as a whole, to reduce the driving force required for the compressor.